Quaternary-containing resin used for magnetic recording media

ABSTRACT

A resin used for magnetic recording media, comprising a vinyl chloride copolymer containing vinyl monomers with hydroxyl groups and vinyl monomers with quaternary ammonium salt groups as component units. The resin is made by a method which comprises polymerizing an organic solvent solution containing vinyl chloride, vinyl monomers with hydroxyl groups and vinyl monomers with quaternary ammonium salt groups in a reacting apparatus to thereby precipitate a vinyl chloride copolymer, the interior surface of said reacting apparatus, which comes into contact with said organic solvent solution being coated with a fluororesin.

This application is a continuation of application Ser. No. 030,457 filedMar. 25, 1987, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a resin having excellent dispersioncharacteristics used for magnetic recording media, which constitutes amagnetic layer that contains magnetic powder, resulting in excellentabrasion resistance, and a method for the production of the resin.

2. Description of the Prior Art

Magnetic recording media used for magnetic videotapes and magneticdisks, etc., are usually obtained by coating, orienting and drying amagnetic coating on a medium made of polyester, etc. The said media musthave excellent magnetic characteristics, electromagnetic transformingcharacteristics and be durable. The magnetic coating includes magneticpowder and a resin by which the magnetic powder adheres to the media.Accordingly, resins used for magnetic recording media must havecharacteristics by which excellent dispersion of magnetic powder can beachieved in a magnetic coating, resulting in a magnetic layer having ahigh squareness ratio, and moreover, by which a magnetic layer havingexcellent abrasion resistance and excellent heat resistance is formed onthe substrate. Especially, in recent years, the development of resinsthat are excellent in dispersion characteristics has become necessarywith the atomization of magnetic powder based on an increased recordingdensity thereof in the magnetic recording media.

As a resin having these excellent characteristics, vinyl chloride -vinyl acetate - vinyl alcohol copolymers (disclosed in, e.g., JapanesePatent Publication No. 61-26132), etc., are used, and when a high levelof abrasion resistance of the magnetic layer is required, a resincontaining the said copolymer and isocyanate compounds is used for theproduction of the magnetic recording media. However, these vinylchloride - vinyl acetate - vinyl alcohol copolymers are inferior indispersion characteristics into the magnetic coating. Vinyl chloridecopolymers to be generally used for magnetic recording media are usuallyproduced by the precipitation polymerization method as a modified methodof the solution polymerization method. According to this precipitationpolymerization method, since the resulting polymer is precipitated, theremoval of the solvent and the residual monomers after thepolymerization is readily carried out and the use of a dispersant and/oran emulsifier is not required. However, when the vinyl chloridecopolymers are produced in a stainless steel reacting apparatus, a partof the copolymers deposits and adheres to the inside of a reactingapparatus, which makes the composition thereof uneven. Moreover, theremoval of the deposited scales from the reacting apparatus istroublesome.

SUMMARY OF THE INVENTION

The resin used for magnetic recording media of this invention, whichovercomes the above-discussed and numerous other disadvantages anddeficiencies of the prior art, comprises a vinyl chloride copolymercontaining vinyl monomers with hydroxyl groups and vinyl monomers withquaternary ammonium salt groups as component units.

In a preferred embodiment, the resin further comprises an isocyanatecompound.

In a preferred embodiment, the content of the vinyl monomers withhydroxyl groups in the vinyl chloride copolymer is in the range of 1 to30% by weight.

In a preferred embodiment, the content of the vinyl monomers withquaternary ammonium salt groups in the vinyl chloride copolymer is inthe range of 0.05 to 8% by weight.

In a more preferred embodiment, the content of the vinyl chloride is inthe range of 60 to 95% by weight, that of the vinyl monomers withhydroxyl groups is in the range of 1 to 30% by weight, and that of thevinyl monomers with quaternary ammonium salt groups is in the range of0.05 to 8% by weight in the vinyl chloride copolymer.

In a preferred embodiment, at least one of the vinyl monomers withquaternary ammonium salt groups is methacryloyloxyethyltrimethylammoniumchloride.

In a preferred embodiment, the vinyl chloride copolymer further containsvinyl monomers with acid radicals as a component unit.

In a more preferred embodiment, the content of the vinyl monomers withhydroxyl groups in the vinyl chloride copolymer is in the range of 1 to30% by weight.

In a further more preferred embodiment, the contents of the vinylmonomers with quaternary ammonium salt groups and the vinyl monomerswith acid radicals are in the range of 0.05 to 8% by weight,respectively, in the vinyl chloride copolymer, their total content beingin the range of 0.1 to 10% by weight.

In a further more preferred embodiment, the content of the vinylchloride is in the range of 60 to 95% by weight, that of the vinylmonomers with hydroxyl groups is in the range of 1 to 30% by weight, andthose of the vinyl monomers with quaternary ammonium salt groups and thevinyl monomers with acid radicals are in the range of 0.05 to 8% byweight, respectively, in the vinyl chloride copolymer, their totalcontent being in the range of 0.1 to 10% by weight.

In a further more preferred embodiment, the vinyl monomers with acidradicals are vinyl monomers having phosphoric groups.

The method for the production of the resin of this invention comprisespolymerizing an organic solvent solution containing vinyl chloride,vinyl monomers with hydroxyl groups and vinyl monomers with quaternaryammonium salt groups in a reacting apparatus to thereby precipitate avinyl chloride copolymer, said interior surface of the reactingapparatus, which comes into contact with said organic solvent solution,being coated with a fluororesin.

In a preferred embodiment, the organic solvent solution further containsmonomers with acid radicals.

The method for the production of vinyl chloride resins of this inventioncomprises polymerizing an organic solvent solution containing vinylchloride, vinyl monomers with hydroxyl groups and vinyl monomers withquaternary ammonium salt groups in a reacting apparatus to therebyprecipitate a vinyl chloride copolymer, the interior surface of saidreacting apparatus, which comes into contact with said organic solventsolution, being coated with a fluororesin.

In a preferred embodiment, the organic solvent solution further containsmonomers with acid radicals.

Thus, the invention described herein makes possible the objects of (1)providing a resin having excellent dispersion characteristics used formagnetic recording media which can constitute a magnetic layer thatcontains magnetic powder, and moreover that has excellent abrasion- andheat-resistance based on an optional addition of an isocyanate compound;(2) providing a resin having excellent dispersion characteristics usedfor magnetic recording media which can constitute a magnetic layer thatcontains magnetic powder and accordingly having a high squareness ratio;(3) providing a resin used for magnetic recording media which comprisesa vinyl chloride copolymer that contains vinyl chloride constituting ahard and flexible magnetic layer in combination with other components;(4) providing a resin to be used for magnetic recording media, whichconsists of a copolymer made of vinyl monomers with hydroxyl groups andvinyl chloride, and when vinyl monomers with quaternary ammonium saltgroups are used as a component of the copolymer, the dispersioncharacteristics of magnetic powder is surprisingly improved; (5)providing a resin used for magnetic recording media in which when theresin comprises a vinyl chloride copolymer containing vinyl monomerswith acid radicals in addition to hydroxyl groups and with quaternaryammonium salt groups, the dispersion of the magnetic powder is furtherimproved due to the multiplied effect between the vinyl monomers withquaternary ammonium salt groups (i.e., with cationic hydrophilic groups)and the vinyl monomers with acid radicals (i.e., with anionichydrophilic groups); (6) providing a resin used for magnetic recordingmedia in which when the resin contains isocyanate compounds, itconstitutes a magnetic layer that is excellent in abrasion- andheat-resistance since the hydroxyl groups incorporated into thecopolymer contribute to the crosslinking reaction of the copolymer withthe isocyanate compounds; (7) providing a method for the production ofthe resin in which the precipitation polymerization method is adopted ina reacting apparatus the interior surface of which is coated withfluororesin, so that deposited scales are not created afterpolymerization, and accordingly there are no dangers that a copolymerhaving an ununiform composition will be produced due to said scales andalso removal of the scales will not be required; and (8) providing amethod for producing vinyl chloride copolymers without the occurrence ofscales by use of the precipitation polymerization method.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The inventors searched for a resin to be used for magnetic recordingmedia, which consists of a copolymer made of vinyl monomers withhydroxyl groups and vinyl chloride, and then realized this inventionupon discovering that if vinyl monomers with quaternary ammonium saltgroups are used as a component of the copolymer, the dispersioncharacteristics of magnetic powder is surprisingly improved and that ifnot only the said vinyl monomers with quaternary ammonium salt groups,which are cationic hydrophilic groups, but also vinyl polymers with acidradicals, which are anionic hydrophilic groups, are used as componentsof the copolymer, according to the multiplication effect of bothfunctional groups, the dispersion characteristics of magnetic powder areexceedingly improved over those in which quaternary ammonium salt groupsor acid radicals are independently used.

This invention was also realized on the basis of the findings of theinventors that the vinyl chloride copolymer can be produced withoutdeposited scales produced by the polymerization of an organic solventsolution of vinyl monomers as a component of the copolymer in a reactingapparatus, the inside of which is coated with a fluororesin.

Vinyl chloride in the vinyl chloride copolymer of this invention resultsin moderate hardness and flexibility of the magnetic layer together withthe other composition units. When the amount of vinyl chloride is toosmall, the abrasion resistance of the magnetic recording media isinsufficient. When the amount of vinyl chloride is excessive, thesolvent solubility thereof decreases. Accordingly, the content of vinylchloride in the said vinyl chloride copolymer is set to be in the rangeof 60 to 95% by weight, preferably.

As vinyl monomers with hydroxyl groups used as a composition unit of thesaid copolymer, there are compounds, the structural formula of which,exhibits those of products of acrylic acid (or methacrylic acid) andpolyhydric alcohols. Some examples of the compounds, i.e., (meth)acrylicester (denoting both acrylic ester and methacrylic ester) are2-hydroxyethyl(meth) acrylate (denoting both 2-hydroxyethylacrylate and2-hydroxyethyl methacrylate), 2-hydroxypropyl-(meth)acrylate,3-chloro-2-hydroxypropyl (meth)acrylate ,polyethyleneglycolmono(meth)acrylate represented by the formula:

    CH.sub.2 ═CR--COO--CH.sub.2 CH.sub.2 O--.sub.n H

(wherein n is an integer of 2 to 9 and R is hydrogen or methyl),polypropyleneglycolmono(meth)acrylate represented by the formula:##STR1## (wherein n is an integer of 2 to 6 and R is hydrogen ormethyl), and 2-hydroxyethyl-2'-acryloyloxyethylphthalate ##STR2##

As vinyl monomers with hydroxyl groups used as a composition unit of theabove-mentioned copolymer, there are also acrylamides ormethacrylamides, an example of which is N-methylol (meth)acrylamide.These are used as a composition unit of the copolymer separately or in asuitable combination thereof. In particular,2-hydroxyethyl(meth)acrylate and 2-hydroxypropyl(meth)acrylate arepreferably used.

When the amount of vinyl monomers with hydroxyl groups used isexcessive, the disadvantages arise that the solvent solubility of thesaid copolymer decreases, and the surface smoothness and moistureresistance of the magnetic recording media decrease. On the other hand,when the amount of vinyl monomers is too small the dispersioncharacteristics of the magnetic powder are decreased. Moreover,insufficient urethane bonding, due to the reaction with an isocyanatecompound contained therein as desired, results in a decrease in thestrength of the coated film and/or a blocking phenomenon in therecording media arises. Therefore, the vinyl monomers in the saidcopolymer are contained in the range of preferably 1 to 30% by weight,and more preferably 8 to 20% by weight.

Vinyl monomers with quaternary ammonium salt groups used in thisinvention mean vinyl monomers containing quaternary ammonium salt groupsin the molecule, preferable examples of which are as follows:2-hydroxy-3-methacryloyloxypropyl trimethylammonium chloride,methacryloyloxyethyl ammonium chloride, methacryloyloxyethyltrimethylammonium chloride, and trimethyl-3-methacrylamide propylammoniumchloride. The vinyl monomers with quaternary ammonium salt groupsgreatly contribute to the improvement of the dispersion characteristicsof the magnetic powder. When the amount of vinyl monomers is excessive,the disadvantages arise that the solvent solubility of the saidcopolymer decreases causing difficulty in obtaining a clear solution,the water resistance decreases, the heat stability decreases, thecrosslinking effect attaining an improvement of the abrasion resistancecannot be achieved due to a side reaction except for the crosslinkingreaction with an isocyanate compound contained, if necessary, and thepot life of the magnetic coating becomes short. On the other hand, whenthe amount of vinyl monomers is too small, the dispersioncharacteristics of the magnetic powder decrease. Therefore, the vinylmonomers with quaternary ammonium salt groups are contained in the rangeof preferably 0.05 to 8% by weight and more preferably 0.1 to 5% byweight in the copolymer. Quaternary ammonium salt groups can beincorporated therein from the polymer containing tertiary amines by thepolymer reaction.

Vinyl monomers with acid radicals in this invention mean vinyl monomerscontaining, for example, phosphoric, carboxyl, sulfonic groups, andtheir metal salts. Monomers with carboxyl groups mean monomerscontaining carboxyl groups in the molecule and having an ability ofvinyl polymerization, suitable examples of which are as follows:compounds having one carboxyl group in the molecule such as acrylicacid, methacrylic acid, monoacryloyloxyethyl succinate ester (CH₂═CHCOO(CH₂)₂ OCO(CH₂)₂ COOH), monoacryloyloxyethyl phthalate ester,monomethacryloyloxyethyl succinate ester, and monomethacryloyloxyethylphthalate ester, and compounds having some carboxyl groups in themolecule such as maleic acid and fumaric acid. Monomers with phosphoricgroups mean monomers containing phosphoric groups in the molecule andhaving an ability of vinyl polymerization, suitable examples of whichare as follows: acid phosphoxyethyl(meth)acrylate, acidphosphoxypropyl(meth)acrylate, and 3-chloro-2-acidphosphoxypropyl(meth)acrylate. Monomers with sulfonic groups meanmonomers containing sulfonic groups in the molecule and having anability of vinyl polymerization, suitable examples of which are asfollows: 2-acrylamide-2-methylpropanesulfonic acid, acryl sulfonic acid,methacrylsulfonic acid, vinylsulfonic acid, and styrenesulfonic acid. Invarious combinations of functional groups, the combination of phosphoricgroups and quaternary ammonium salt groups is particularly suitable fromthe viewpoint of an improvement in the dispersion characteristics ofmagnetic powder. Quaternary ammonium salt groups contribute to animprovement in the dispersion characteristics of magnetic powder due toa multiplied action together with acid radicals. In the copolymer, vinylmonomers with quaternary ammonium salt groups and vinyl monomers withacid radicals are contained in the range of 0.05 to 8% by weight,respectively. The total content of both monomers is in the range ofpreferably 0.1 to 10% by weight and more preferably 0.1 to 5% by weight.

The resin of this invention can contain monomers such as ethylene,propylene, and vinyl acetate as a composition unit of the vinyl chloridecopolymer as desired. Homopolymers or copolymers of the above-mentionedmonomers can be used together with the vinyl chloride copolymer. The useof ethylene as a composition unit of the vinyl chloride copolymer isparticularly suitable from the viewpoint of improvement of the solventsolubility of the vinyl chloride copolymer.

The vinyl chloride copolymer mentioned above can be produced by a knownmethod of polymerization, e.g., the precipitation polymerization method,the solution polymerization method, the suspension polymerizationmethod, and the emulsion polymerization method. The precipitationpolymerization method is particularly preferable since neither adispersant nor an emulsifier is required and since removal of thesolvent and/or the residual monomers after polymerization is readilycarried out. In this precipitation polymerization method, in order toprevent the occurrence of deposited scales on the inside of the reactingapparatus, the part of the interior surface of the reacting apparatus,which comes into contact with the monomer solution, is coated with afluororesin.

As the solvent used for polymerization, organic solvents, which dissolvevinyl chloride, the above-mentioned monomer, having an ability ofpolymerization and a polymerization initiator therein, and whichseparates the produced copolymer therefrom, are used. Examples of theseorganic solvents are as follows: aliphatic alcohols such as methanol,ethanol, and butanol; saturated aliphatic hydrocarbons such as propane,butane, pentane, and hexane; saturated alicyclic hydrocarbons such ascyclobutane. The number of carbon atoms in the aliphatic alcohol isdesired to be 1 to 5, and that in the saturated aliphatic and thesaturated alicyclic hydrocarbon is desired to be 3 to 8 from theviewpoint of the solubility of the monomer. These organic solvents canbe used as a mixture of more than two kinds of solvents. They can be, ofcourse, of a mixture of alcohol and water. The selection thereof isperformed depending upon the nature of the monomers used. Since maleicacid and 2-acrylamide-2-methylpropanesulfonic acid are not dissolvableinto a hydrocarbon solvent such as n-hexane, they cannot be used. Analcohol solvent such as methanol can be used. Especially, methanol issuitable because it dissolves vinyl monomers with quaternary ammoniumsalt groups therein at a high level. Since some monomers cause achemical reaction with solvents in the same manner as that in thesuspension polymerization method, close attention must be paid to thechoice of monomers.

In contrast to water in which the solubility of monomers are poor, evenwhen vinyl chloride and/or monomers having an ability of polymerizationare added to a solution containing a solvent during polymerization, themonomer can be quickly dissolved, resulting in a solution having auniform composition. Therefore, when the copolymer is prepared withmonomers having an ability of polymerization, e.g., (meth)acrylic ester,the monomer reactivity ratio of which is larger than that of vinylchloride, the composition of each monomer in the reacting system can bemaintained to be constant during polymerization, if this solution isproperly added according to the progress of the reaction conditions. Theresulting copolymer has a uniform composition.

Polymerization is performed using the abovementioned monomers, whichbecome a composition unit of the vinyl chloride copolymer, an initiatorand the above-mentioned organic solvent. As the initiator, organicperoxides or azo compounds, which are usually used for thepolymerization, are employed.

As the fluororesin used for coating of the reacting apparatus, anyproduct which is available on the market can be used, examples of whichare as follows: polytetrafluoroethylene (teflon TFE; Dupon Co., Ltd.),tetrafluoroethylene-hexafluoropropylene copolymer (teflon FEP; DuponCo., Ltd.), ethylenetetrafluoroethylene copolymer (aflon COP; AsahiGlass Co., Ltd.), tetrafluoroethylene-perfluoroalkoxyethylene copolymer(PFA; Mitsui Fluorochemical Co., Ltd.), polytrifluoroethylene chloride(CTFE), and polyfluorovinylidene (PVdF). The inside of the reactingapparatus which comes into contact with the reacting solution is usuallycoated with these powdered fluororesins and sintered. The thickness ofthe coated film is desired to be in the range of 0.1 to 1.0 mm. The useof the reacting apparatus coated with a fluororesin provides very littleadhesion of deposited scales to the apparatus and separation of theproduced copolymer as a powdered precipitate. Thus, the disadvantagesthat the copolymer cannot be prepared with a uniform composition and theinitiator is incorporated into the scale and cannot function as aninitiator are removed. The separated copolymer can be readily removedfrom the reacting apparatus. The inside of the reacting apparatus can becoated with a fluororesin as desired, not only to provide additionalstrength, but also to line the reacting apparatus and function as apacking material. Especially, ethylenetetrafluoroethylene copolymer andtetrafluoroethyleneperfluoroalkoxyethylene copolymer can produce a thickand tenacious film on the interior surface of a reacting apparatus. Bythe powder coating technique, a fluororesin film can be readily formedon the interior surface of the reacting apparatus, even one having acomplicated shape. Since such a fluororesin film is excellent inresistance to organic solvents, resistance to acids, resistance toalkalies, etc., it will not corrode, no matter what solvents andmonomers are used in the method of this invention.

Such a reacting apparatus coated with a fluororesin can effectivelyprevent deposited scales from arising in the solution polymerizationmethod, especially in the precipitation polymerization method. Whencopolymerization is performed by the suspension polymerization method inthis reacting apparatus, a large amount of deposited scales arises andadheres to the apparatus.

According to this precipitation polymerization method, the copolymer canbe obtained generally in a fine-powdered form. The average degree ofpolymerization in the copolymer is preferably in the range of about 150to about 600 from the viewpoint of both the mechanical strength of thebinder and the characteristics of the magnetic coating. If the averagedegree of polymerization is less than 150, when this copolymer is coatedon a substrate together with magnetic powder, the copolymer film (i.e.,a magnetic layer) obtained becomes weak, causing difficulties inpractical use. When the average degree of polymerization is over 600,the solution viscosity of this copolymer is so high that the efficiencyof the coating process is decreased.

Some examples of isocyanate compounds, which are kneaded with magneticpowder together with the above-mentioned copolymer when the abrasionresistance and heat-resistance of the resulting magnetic layer areraised, are as follows: tolylenediisocyanate,diphenylmethanediisocyanate, dianisidinediisocyanate,hexamethylenediisocyanate, methaxylylenediisocyanate, and a product of 1mol of trimethylolpropane with 3 mol of tolylenediisocyanate. Thisproduct is available from, for example, Japan Polyurethane Industry Co.,Ltd., under the name of "Coronate L".

When the amount of isocyanate compounds used is excessive, thecrosslinking density increases so that the coated film obtained finallyhardens and becomes brittle. When the amount of isocyanate compounds istoo small, the expected effects cannot be attained. Therefore, the saidamount is usually set to be in the range of 0.3 to 30 parts by weightfor 100 parts by weight of the above-mentioned copolymer.

The production of the magnetic coating from the resin of this inventionis performed as follows: For example, the above-mentioned vinyl chloridecopolymer is dissolved in a mixed solvent of methylisobutyl ketone andtoluene. To the resulting solution, magnetic powder material such asγ-iron oxide is added together with an additive such as surfactant,kneaded, and dispersed. When an increase in the abrasion resistance ofthe magnetic layer is particularly important, an isocyanate compound isadded to this solution, resulting in a magnetic coating.

However, the order of addition and the method of dispersion of theabove-mentioned copolymer, isocyanate compound and magnetic powderedmaterial in this invention are not limited. The solvent which dissolvesthe above-mentioned copolymer so as to produce the resin of thisinvention is one selected from the groups consisting of toluene,methylethyl ketone, methylisobutyl ketone, and cyclohexanone, or amixture thereof. The copolymer is usually used in the range of 10 to 100parts by weight for 100 parts by weight of the magnetic powderedmaterial. The magnetic coating is usually prepared so as to contain thecopolymer at a concentration of 5 to 30% by weight.

EXAMPLES Squareness ratio

Magnetic coating to which isocyanate compounds had been added as desiredwas coated on a polyester film as a substrate of 25 μm thickness so asto form a magnetic film with a dry thickness of 6 μm, oriented, anddried, resulting in a magnetic recording medium, the squareness ratio(Br/Bm, where Br and Bm denote the residual flux density and thesaturation flux density, respectively) of which was then measured. Whenthe squareness ratio is around 0.8 or more, the dispersion of themagnetic powder becomes remarkably high.

Degree of Gloss

The reflective index of rays of light that were incident upon themagnetic film at an angle of 60, which had been subjected to themeasurement of the squareness ratio, was measured by a glossmeter andthe degree of gloss was evaluated on the basis of the obtained value.The degree of gloss exhibits not only smoothness on the surface of themagnetic film but also an index of the dispersion characteristics of themagnetic powder in the magnetic film.

Thermocontact

In order to evaluate the thermocontact characteristics, examinationswere carried out to check whether or not the magnetic layers adhere toeach other when two magnetic recording media were heated at atemperature of 120° C. for 15 min and then overlapped so as to have themagnetic layers face each other. In Table 1, O denotes non-adhesiveness(blocking does not occur) and X adhesiveness.

Gel fraction

A vinyl chloride copolymer film which contains no magnetic powder wasimmersed overnight in a mixed solvent of toluene-methylisobutylketone(weight ratio 1:1) at 50° C. The value obtained by dividing the weightof the immersed film by the weight of a film that has not yet beenimmersed was considered to be a gel fraction, which is an index ofefficiency, etc., of the crosslinking reaction between the copolymer andthe isocyanate compounds.

EXAMPLE 1 Initial preparation

The interior surface of an autoclave with an agitator, the volume ofwhich is 20 , l the agitation wings and the surface of the baffle werecoated with tetrafluoroethylene-perfluoroalkoxyethylene copolymer (PFA;Mitsui Chlorochemical Co., Ltd.) and sintered to form a coated filmthereon. After the autoclave with the interior surface coated with thefluororesin was charged with 8100 g of methanol (first grade reagent)and 25 g of α-cumylperoxyneodecanoate as a polymerization initiator, theair inside of the autoclave was removed by evacuation using an aspiratorfor 5 min. The autoclave was then charged with 2600 g of vinyl chloride.

Post-preparation

After the air inside of a stainless vessel, which can be hung, with thevolume of 3 l (referred to as a feeding vessel) was removed, a mixedsolution of 259 g of 2-hydroxypropylacrylate dissolved in 500 g ofmethanol and 15 g of methacryloyloxyethyltrimethylammonium chloride wascharged into the vessel, and then 1045 g of vinyl chloride was injectedin this vessel.

This vessel was shaken so as to mix and dissolve the contents and thenhung on a spring scale. Then, it was connected to a nozzle of theautoclave through a valve at the bottom of the vessel using a flexibletube.

Polymerization treatment

The number of rotations of the agitator of the autoclave was set to be380 rpm and the temperature inside of the autoclave was increased to be43° C. by hot water flowing through the jacket. Polymerization wasinitiated as soon as the internal temperature rose to 43° C., andaccordingly the internal temperature was maintained to be 43° C. for thesucceeding polymerization.

When the internal temperature attained 43° C., 50 g of a mixed monomersolution was supplied to the autoclave from the feeding vessel, and thena certain amount of the solution was supplied in installments, that is,four times of 46 g each at 5 min. intervals, eight times of 31 g each at10 min. intervals, or forty nine times of 27 g each at 5 min. intervals,depending upon the progress of polymerization. Ten minutes after thelast addition, the autoclave was cooled to 25° C. and the polymerizationwas terminated. The pressure inside of the autoclave was 2.0 kg/cm² G atthe initial stage and 1.8 kg/cm² G at the final stage.

After cooling, unreacted vinyl chloride was degassed and substantiallyremoved by the feeding of nitrogen gas thereinto, and then methanolslurry of the copolymer was removed from the autoclave, followed byfiltration and drying under reduced pressure at 50° C. for 24 hours,resulting in 1200 g of white powdered copolymer A.

The resulting resin had a polymerization degree of 300 and was composedof 86.5% of vinyl chloride, 13.0% of 2-hydroxypropylacrylate, and 0.5%of methacryloyloxyethyltrimethylammonium chloride. After thepolymerization, adhesion of deposited scales to the interior surface ofthe reacting apparatus, the stirring wings and the surface of the bafflewas not observed. Dissolution of the resin in a mixed solvent ofmethylisobutyl ketone-toluene (1:1) at 10% by weight afforded a clearsolution without residue.

The copolymer A was dissolved in a solution of toluene and methylisobutyl ketone (weight ratio 1:1) to prepare a copolymer solutioncontaining the copolymer A in a concentration of 15%. To this copolymersolution, 80 parts by weight of Co-γ-iron oxide (magnetic powder)per 20parts by weight of the copolymer dissolved in the solution and stainlessballs having a diameter of 1/8 inches in an amount eight times that ofthe magnetic powder were added and mixed by a paint conditioner(Reddevil Co., Ltd.) for 6 hours to provide a magnetic coating, followedby preparing a magnetic recording medium, the squareness ratio of whichwas then determined to be as high as 0.82. The degree of gloss thereofwas determined to be 96%.

EXAMPLE 2

The reacting apparatus, in which the interior surface, agitating wingsand the baffle surface were coated with ethylene-tetrafluoroethylenecopolymer (aflon COP; Asahi Glass Co., Ltd.), was charged with the samemonomers as those in Example 1 in the same manner as in Example 1 andpolymerization was carried out in the same manner as in Example 1,wherein the amounts of monomers, methanol and initiator were changed. Apowdered copolymer B was obtained, first, and the composition of thiscopolymer was then analyzed. Then, a magnetic recording medium wasprepared in the same manner as in Example 1 and the squareness ratiothereof was determined, the results of which are shown in Table 1.Moreover, as a fluororesin, tetrafluoroethylene-hexafluoropropylenecopolymer (teflon FEP; Dupon Co., Ltd.) was used in place ofethylene-tetrafluoroethylene copolymer and, in the same manner as in thepreparation of the copolymer B, copolymers C, D, E, F, and G, thecomposition of each of which is indicated in Table 1, were obtained toprepare the magnetic recording media corresponding thereto. Theirsquareness ratios are greater than 0.80 as shown in Table 1 and thedegrees of gloss are greater than 90%.

In any case, adhesion of deposited scales to the interior surface of thereacting apparatus, the agitating wings and the surface of the bafflewas not substantially observed. The solvent solubility test was carriedout in the same manner as in Example 1, resulting in a colorless clearsolution.

                                      TABLE 1                                     __________________________________________________________________________                       Copolymers                                                                    B       C      D      E      F      G                      __________________________________________________________________________    Copolymer Compositions (parts)                                                Vinyl Chloride     85.9    84.5   86.3   85.5   85.7   79.8                   Vinyl Monomer with Hydroxyl Group                                             2-Hydroxyethylacrylate            13.1          13.4                          2-Hydroxypropylacrylate                                                                          13.0    13.4          13.5          17.3                   Vinyl Monomer with Quarternary                                                Ammonium Salt Group                                                           Methacryloyloxyethyltrimethyl-                                                                   1.1     2.1    0.6                  2.9                    ammonium Chloride                                                             Trimethyl-3-methacrylamide-                                                   propylammonium Chloride                  1.0    0.9                           Average Degree of Polymerization                                                                 310     300    310    310    300    300                    Physical                                                                            Squareness Ratio                                                                           0.82    0.82   0.83   0.83   0.83   0.82                   Property                                                                            Degree of Gloss (%)                                                                        97      94     96     96     95     95                     Adhesion of Scales None    None   None   None   None   None                   Solubility Test    Completely                                                                            Completely                                                                           Completely                                                                           Completely                                                                           Completely                                                                           Completely                                dissolved                                                                             dissolved                                                                            dissolved                                                                            dissolved                                                                            dissolved                                                                            dissolved                                 in a Solvent                                                                          in a Solvent                                                                         in a Solvent                                                                         in a Solvent                                                                         in a Solvent                                                                         in a                   __________________________________________________________________________                                                           Solvent            

EXAMPLE 3

A copolymer H was obtained in the same manner as in Example 1 exceptthat as the succeeding preparation process, a mixed solution, preparedby dissolving 244 g of 2-hydroxypropylacrylate, 15 g ofmethacryloyloxyethyltrimethylammonium chloride, and 15 g of acidphosphoxyethylmethacrylate in 500 g of methanol, was charged into thestainless vessel and then 1045 g of vinyl chloride was injected into thevessel under pressure.

The resulting resin had a polymerization degree of 300 and was composedof 86.5% of vinyl chloride, 12.5% of 2-hydroxypropylacrylate, and 0.5%of methacryloyloxyethyltrimethylammonium chloride, and 0.5% of acidphosphoxyethylmethacrylate. After the polymerization, adhesion of thescales to the interior surface of the reacting apparatus, the stirringwings and the surface of the baffle was not observed. Dissolution of theresin in a mixed solvent of methylisobutyl ketone-toluene (1:1) in the10% by weight afforded a clear solution without residue.

The copolymer H was dissolved in a mixed solvent of toluene and methylisobutyl ketone (weight ratio 1:1) to prepare a copolymer solutioncontaining the copolymer A in a concentration of 15%. To this copolymersolution, 80 parts by weight of Co-γ-iron oxide (magnetic powder per 20parts by weight of the copolymer dissolved in the solution and stainlessballs having a diameter of 1/8 inches in an amount eight times that ofthe magnetic powder were added and mixed by a paint conditioner(Reddevil Co., Ltd.) for 6 hours to provide a magnetic coating, followedby preparing a magnetic recording medium, the squareness ratio of whichwas then determined to be as high as 0.84. The degree of gloss thereofwas determined to be 108%.

EXAMPLE 4

The reacting apparatus, in which the interior surface, agitating wingsand the baffle surface were coated with ethylene-tetrafluoroethylenecopolymer (aflon COP; Asahi Grass Co., Ltd.), was charged with the samemonomers as those in Example 3 in the same manner as in Example 3 andpolymerization was carried out in the same manner as in Example 1,wherein the amounts of monomers, methanol and initiator were changed. Apowdered copolymer I was obtained, first, and the composition of thiscopolymer was then analyzed. Then, a magnetic recording medium wasprepared in the same manner as in Example 1 and the squareness ratiothereof was determined, the results of which are shown in Table 2.Moreover, as a fluororesin, tetrafluoroethylene-hexafluoropropylenecopolymer (teflon FEP; Dupon Co., Ltd.) was used in place ofethylene-tetrafluoroethylene copolymer and, in the same manner as in thepreparation of the copolymer B, copolymers J, K, L, and M, thecomposition of each of which is indicated in Table 2, were obtained toprepare the magnetic recording media corresponding thereto. Theirsquareness ratios are greater than 0.80 as shown in Table 2 and thedegrees of gloss are greater than 100%.

In any case, adhesion of deposited scales to the interior surface of thereacting apparatus, the agitating wings and the surface of the bafflewas not substantially observed. The solvent solubility test was carriedout in the same manner as in Example 1, resulting in a colorless clearsolution.

                                      TABLE 2                                     __________________________________________________________________________                       Copolymers                                                                    I      J      K      L      M                              __________________________________________________________________________    Copolymer Compositions (parts)                                                Vinyl Chloride     85.0   83.8   84.3   85.6   82.9                           Vinyl Monomer with Hydroxyl Group                                             2-Hydroxypropylacrylate                                                                          13.0   13.0   13.5   13.2   13.1                           Vinyl Monomer with Quaternary                                                 Ammonium Salt Group                                                           Methacryloyloxyethyltrimethyl-                                                                   1.0    1.6    1.1    0.6    2.0                            ammonium Chloride                                                             Vinyl Monomer with Acid Radical                                               Acid Phosphoxyethylmethacrylate                                                                  1.0    1.6                  2.0                            Maleic Acid                      1.1    0.6                                   Average Degree of Polymerization                                                                 340    350    340    360    330                            Physical                                                                            Squareness Ratio                                                                           0.85   0.84   0.83   0.83   0.84                           Property                                                                            Degree of Gloss (%)                                                                        110    108    105    104    110                            Adhesion of Scales None   None   None   None   None                           Solubility Test    Completely                                                                           Completely                                                                           Completely                                                                           Completely                                                                           Completely                                        dissolved                                                                            dissolved                                                                            dissolved                                                                            dissolved                                                                            dissolved                                         in a Solvent                                                                         in a Solvent                                                                         in a Solvent                                                                         in a Solvent                                                                         in a Solvent                   __________________________________________________________________________

EXAMPLE 5

The copolymer A obtained in Example 1 and the copolymers B to G obtainedin Example 2 were dissolved, respectively, in a mixed solvent oftoluenemethylisobutyl ketone (weight ratio 1:1), resulting in copolymersolutions in concentration of 15%.

To each of the copolymer solutions, 80 parts by weight of Co-γ-ironoxide (magnetic powder) per 20 parts by weight of the copolymerdissolved in the solution and stainless balls having a diameter of 1/8inches in an amount eight times that of the magnetic powder were addedand mixed by a paint conditioner for 6 hours, and then 0.65 parts byweight of an isocyanate compound (Coronate L, Japan PolyurethaneIndustry Co., Ltd.) were added thereto and stirred, resulting in amagnetic coating corresponding to each of the copolymers A to G. Thesecoatings were applied to polyester films, aligned, and dried to preparemagnetic recording media in which the crosslinking reaction hadsubstantially proceeded. The squareness ratios, the degrees of gloss,and thermocontact characteristics of these magnetic recording media areshown in Table 3. Gel fractions, which represented the crosslinkingreactivity between each copolymer and the isocyanate, are also shown inTable 3.

                  TABLE 3                                                         ______________________________________                                        Copolymers A       B      C    D    E    F    G                               ______________________________________                                        Squareness Ratio                                                                         0.82    0.81   0.82 0.82 0.81 0.82 0.81                            Degree of Gloss                                                                          95      97     95   96   93   95   96                              (%)                                                                           Thermocontact                                                                            ○                                                                              ○                                                                             ○                                                                           ○                                                                           ○                                                                           ○                                                                           ○                        Gel Fraction (%)                                                                         81      82     82   80   81   81   92                              ______________________________________                                    

EXAMPLE 6

The copolymer H obtained in Example 3 and the copolymers I to M obtainedin Example 4 were dissolved, respectively, in a mixed solvent oftoluenemethylisobutyl ketone (weight ratio 1:1), resulting in copolymersolutions in concentration of 15%.

To each of the copolymer solutions, 80 parts by weight of Co-γ-ironoxide (magnetic powder) per 20 parts by weight of the copolymerdissolved in the solution and stainless balls having a diameter of 1/8inches in an amount eight times that of the magnetic powder were addedand mixed by a coating conditioner for 6 hours, and then 0.65 parts byweight of an isocyanate compound (Coronate L, Japan PolyurethaneIndustry Co., Ltd.) were added thereto and stirred, resulting in amagnetic coating corresponding to each of the copolymers H to M. Thesecoatings were applied to polyester films, aligned, and dried to preparemagnetic recording media in which the crosslinking reaction hadsubstantially proceeded. The squareness ratios, the degrees of gloss,and thermocontact characteristics of these magnetic recording media areshown in Table 4. Gel fraction, which represented the crosslinkingreactivity between each copolymer and the isocyanate, are also shown inTable 4.

                  TABLE 4                                                         ______________________________________                                        Copolymers H      I       J    K     L    M                                   ______________________________________                                        Squareness Ratio                                                                         0.83   0.83    0.84 0.82  0.83 0.83                                Degree of Gloss                                                                          110    108     109  105   110  105                                 (%)                                                                           Thermocontact                                                                            ○                                                                             ○                                                                              ○                                                                           ○                                                                            ○                                                                           ○                            Gel Fraction (%)                                                                         84     83      85   82    84   83                                  ______________________________________                                    

Control 1

Reference standard magnetic recording media were prepared, in the samemanner as in the above-mentioned examples, using a vinyl chloride /2-hydroxypropylacrylate copolymer a (the average degree ofpolymerization of which was 350 and which contained 13.0% by weight of2-HPA) which were produced in the same manner as in the preparation ofthe resin A of Example 1, and a vinyl chloride/vinyl acetate/vinylalcohol copolymer b (the composition ratio of which was 91/3/6 in thisorder). The squareness ratios of the magnetic recording media weredetermined to be 0.75 and 0.76, respectively. The degrees of glossthereof were as low as 52% and 54%, respectively.

Control 2

In respect to the copolymer b used in Control 1, a reference standardmagnetic recording medium was prepared in the same manner as in Example2 and was subjected to a thermocontact test, which resulted in ablocking phenomenon.

Control 3

A vinyl chloride copolymer was prepared in the same manner as in Example1 except for the use of a reacting apparatus, the interior surface ofwhich was not coated with a fluororesin. It was observed that depositedscales having a thickness of about 5 mm adhered to the interior surfaceof the reacting apparatus. It was also observed that the scales adheredto the surface of the baffle as well as to the agitating wings. Thesedeposited scales were scraped away with a scraper and were subjected toa solvent solubility test, which resulted in a cloudy solution,indicating that a portion of deposited scales is not solvent-soluble.

It is understood that various other modifications will be apparent toand can be readily made by those skilled in the art without departingfrom the scope and spirit of this invention. Accordingly, it is notintended that the scope of the claims appended hereto be limited to thedescription as set forth herein, but rather that the claims be construedas encompassing all the features of patentable novelty that reside inthe present invention, including all features that would be treated asequivalents thereof by those skilled in the art to which this inventionpertains.

What is claimed is:
 1. A magnetic recording medium comprising anon-magnetic substrate on which is coated a layer of a dispersion of aparticulate magnetic material in a vinyl chloride copolymer containing,as component units, 60-95% by weight of vinyl chloride, 1 to 30% byweight of vinyl monomers with hydroxyl groups, and 0.05% to 8% by weightof vinyl monomers with quaternary ammonium salt groups.
 2. A magneticrecording medium according to claim 1, wherein the content of the vinylmonomers with hydroxyl groups in the vinyl chloride copolymers is in therange of 8 to 20% by weight.
 3. A magnetic recording medium according toclaim 1, wherein the content of the vinyl monomers with quaternaryammonium salt groups in the vinyl chloride copolymer is in the range of0.1 to 5% by weight.
 4. A magnetic recording medium according to claim1, wherein at least one of the vinyl monomers with quaternary ammoniumsalt groups is methacryloyloxyethyltrimethylammonium chloride.